1. Field of Invention
The present invention relates generally to interposers used in integrated semiconductor packages, and more particularly to a mold and method for fabricating interposers having improved structural and electrical integrity.
2. Description of Prior Art
Developments in integrated chip packages have substantially increased the amount of wiring required for input/output connections, while the sizes of the packages have decreased dramatically. Electrical leads for the I/O connections placed around the perimeter of the chip are restricted in number by the limited space around the ever shrinking chip. One method of handling the increase in number of I/O connections has been to replace the electrical leads with electrical contacts mounted on the surface of the chip instead of the contacts extending around the perimeter.
Several problems arise when a chip with surface mounted electrical contacts is brought in direct contact with a circuit board. For example, soldering the chip directly to the circuit board makes replacement infeasible. Furthermore, the complexity of the wiring leads to cost prohibitive methods of manufacturing. To solve the various problems, a device was designed to interconnect the chip with the circuit board. These interconnect devices are commonly referred to as “interposers.” Interposers offer the advantages of self-centering during assembly, accommodation of higher I/O connections, and enhanced chip performance.
A common example of an interposer is a ductile direct electrical interconnecting device comprising a non-conductive carrier sheet with conductive interconnect members positioned in spaced relation to one another mounted therein. The interconnect members are arranged to individually contact the surface mounted I/O connections and extend through the insulator sheet to contact the electrical contacts of a circuit board on the other side. The interconnect members are typically composed of conductive particles suspended in an elastomer or epoxy and may contain a variety of spherical and non-spherical particles formed from conductive metals.
Interposers are sometimes formed by an injection molding process. FIG. 1 illustrates a typical prior art mold 100 useful for forming interposers. Mold 100 comprises an upper mold section 102 and a lower mold section 104 that sandwiches a non-conductive carrier sheet 114 with passages 106 formed therethrough. Upper mold section 102 includes an entrance via 108 that provides a passage into the upper and lower mold cavities 110, 112, which form the upper and lower contact faces of the interconnect members 116, respectively.
In the typical injection method, an elastomeric mixture containing the conductive particles is injected through via 108 where it then passes through passages 106 in non-conductive carrier sheet 114 and stops in the lower half of the mold. As a result of the injection of the conductive elastomer into the mold cavity, any air contained within the mold is displaced. This displacement of the air within the mold cavity results in venting of the air along the intersection of the two mold sections as it is the primary avenue for escape. The venting along the non-conductive carrier sheet allows some of the elastomer mixture to escape along the non-conductive carrier sheet, which results in a condition commonly referred to in the art as “mold flash.” The occurrence of mold flash prevents the use of smaller and more electrically efficient materials in constructing the interconnect elements, thus negatively affecting the potential electrical performance of the interposer.
Due to the flow pattern of the elastomeric mixture and flow stoppage in the lower section of the mold, the conductive particles often settle and become positioned horizontally. This settling results in a loss of conductivity at the contact surface of the newly formed interconnect members due to the conductive particles not extending through the end of the elastomer.
3. Objects and Advantages
It is a principal object and advantage of the present invention to provide an interposer with interconnect members having enhanced electrical performance.
It is an additional object and advantage of the present invention to provide a method for making interposer interconnect members with improved conductive particle distribution.
It is a further object and advantage of the present invention to provide a method of making interposers with less flashing or loss of elastomeric material.
Other objects and advantages of the present invention will in part be obvious, and in part appear hereinafter.